Don't require config file for supervisorctl

Second try. I closed my previous pull request (#71) because I discovered a bug when executing `supervisord` and `supervisorctl`. In a way, it surprised me that existing tests did not catch the fact that `supervisorctl` could not be executed without arguments. There's a test for it now. However, not for `supervisord`.

I think the title for this pull request says it all. If not, have a look at issue #62. Please refer to the individual commits for a description of what changes have been made. Holler if you would like me to squash the commits into a single monolithic change and make a new pull request.

This commit fixes issue #62.

All tests passes on my workstation.